Due to its excellent functional characteristics, electroless gold plating has been widely used in the field of electronic industry where plating of complex fine circuits, isolated parts with difficult access to leads, etc. is needed. As common electroless gold plating solutions there have been heretofore used those containing. cyanides of strong toxicity as a gold (I) ion-complexing agent, which are usually used at high temperatures under strongly alkaline conditions. As a result, where they are used in such applications as mentioned above, various problems are encountered, for example, separation of resists used for masking circuits or corrosion of ceramic base materials by alkalis. Furthermore, cyanide-containing electroless gold plating baths are extremely high in toxicity and thereby cause problems in their handling, storing and controlling as well as with regard to the safety of working environments and the economy of waste liquid disposal.
On the other hand, as a representative of gold plating solutions using no cyanides there has been known a gold plating solution using a chloroaurate (III) as a gold source (see, for example, U.S. Pat. No. 4,142,902 and GB, A, 114159). This chloroaurate (III) based gold plating solution, where a gold complex salt is formed from the chloroaurate (III) and a sulfite or thiosulfate as a further ingredient, has been put into practical use as cyanide-free plating solution.
In addition those plating solutions using sodium chloroaurate (III) or sodium gold (I) thiosulfate as a gold source and containing sodium sulfite and thiourea as further ingredients have been known from U.S. Pat. Nos. 4,804,559 and 4,880,464. With the plating solutions described in these U.S. Patents, plating, although possible under weakly acidic conditions, proceeds at a low rate and high temperatures are required to raise the plating rate. Furthermore, although continuous plating is possible therewith, plating has to be performed for a long period of time even at a temperature of as high as 80.degree. C. if a film thickness of 5 .mu.m or more is to be achieved.
The present inventors have previously provided an improvement in electroless gold plating solutions with the above-described chloroaurate (III) as a gold source, which improvement comprises using ascorbic acid as a reducing ingredient (see Japanese published unexamined patent application No. 1-191782, JP, A, 1-191782). These improved electroless gold plating solutions containing chloroauric (III) acid or a salt thereof, an alkali metal or ammonium sulfite or thiosulfate and ascorbic acid or a salt thereof as ingredients provide practical plating rates at low temperatures under approximately neutral pH conditions. Also, they are advantageous in that they can be used as a gold plating solution for fine circuits or leads on printed plate boards etc. without causing corrosion of ceramic substrates or separation of masking resists.
Even these electroless gold plating solutions, however, are not satisfactory, since they present safety problems yet to be solved, for example, formation of small quantities of precipitate during their use or precipitation of fine particles of gold during their storage subsequent to their make up. Reasons for the instability of those gold plating baths using a sulfite gold (I) complex salt or thiosulfate gold (I) complex salt as a gold source as well as of those gold plating baths where a gold complex is formed in situ from a chloroaurate (III) and sulfurous or thiosulfuric acid have not been demonstrably elucidated but might be as follows:
Thus, for example, it might be that autoxidation and concentration reduction during storage or plating of such easily oxidizable ingredients as sulfite or thiosulfate ions lead to a change in the equilibrium state of the solution to instabilize the gold complex with the result that the gold activity is increased to render the bath liable to be decomposed. Furthermore, it might be that possible contamination with traces of such metal ions as would enhance the oxidizing activity of the ascorbic acid lead to formation of fine particles of gold with these ions as nuclei, which in turn accelerates decomposition of the bath.